Ever Wonder Why Caffeine Works for Everyone Except You? Here’s Why

There’s a familiar scene that plays out every morning in offices, kitchens, and coffee shops: one person can finish a third cup before noon and still feel sluggish, while someone else gets restless halfway through a single latte.

If caffeine has ever seemed to pass right through you, leaving you wondering why your coffee does so little, you’re not imagining it.

A new study published in the peer-reviewed journal Brain and Behavior offers a new piece of the puzzle. Researchers discovered that caffeine responses differed among individual zebrafish, with anxiety-like reactions varying alongside biological traits such as sex and adenosine-related gene activity.

The findings add to growing evidence that biology shapes caffeine sensitivity. The reason your morning coffee feels powerful, pointless, or unpredictable may be more complicated than the drink itself.

What Caffeine Is Actually Doing in Your Brain

Most people think caffeine gives them energy. It doesn’t, at least not in any direct sense.

Instead, caffeine blocks adenosine, a chemical that builds up in the brain throughout the day and contributes to tiredness. By occupying adenosine receptors, caffeine reduces the brain’s ability to register that sleep pressure. It does not create new energy; it temporarily hides the signal that your body needs rest.

That distinction helps explain why caffeine can feel ineffective after a poor night’s sleep. When adenosine levels are already high, caffeine may not be strong enough to cover the fatigue underneath completely.

Once the caffeine wears off, the accumulated adenosine can return with the familiar crash many people experience later in the day.

The Six-to-Twenty-Hour Variable

How quickly your body processes caffeine is where the biology gets genuinely surprising.

Caffeine can take anywhere from six to twenty hours to clear from the body, depending on the individual, according to Dr. Marilyn Cornelis, an associate professor of nutrition at Northwestern University. That means the same afternoon espresso that barely affects one person could still be active in another person’s system when they are trying to sleep.

Much of this difference comes down to a liver enzyme called cytochrome P450 1A2, produced by the CYP1A2 gene, that helps break caffeine down. People with faster-metabolizing variants remove caffeine more quickly, while those with slower variants keep it circulating longer.

Dr. Cornelis explains to the BBC how this can create delayed effects: “It’s possible to have a first cup of coffee and maybe not feel the effects. But then two or three hours later, the caffeine is still lingering in the system, and, if you add a second cup, that might be enough to tip the scales, and you start feeling a little anxious.”

For some people, the first cup is only the beginning.

When the Body Adjusts

Regular coffee consumers often notice a frustrating pattern: the same dose starts doing less over time.

This is tolerance. With repeated caffeine exposure, the brain can respond by creating more adenosine receptors. As a result, the same amount of caffeine affects fewer of those receptors, making the sensation less noticeable.

That is why someone who drinks coffee every day may tolerate amounts that would leave a less frequent drinker feeling overwhelmed. The body has adjusted to the repeated interruption of the adenosine signal.

However, people who suddenly stop heavy caffeine use may experience headaches, tiredness, and trouble concentrating. Those symptoms reflect the body adapting again as the adenosine system returns to normal without caffeine blocking it.

Related: Coffee and Confidence: How Your Brew Shapes Your Day

Smoking, Medications, and Pregnancy Change the Equation

Several behavioral and physiological factors can meaningfully shift how long caffeine stays in your system.

Smoking is one of the strongest examples. Research has found that smoking can speed up caffeine metabolism, allowing the body to remove caffeine faster. When smokers quit, that effect reverses, and their usual coffee intake may suddenly feel much stronger because caffeine remains in the body longer.

Medications can also alter the equation. Some antibiotics, contraceptive pills, and prescription drugs can slow caffeine clearance, allowing it to accumulate even when a person’s intake has not changed.

Pregnancy also affects caffeine metabolism. Hormonal changes can reduce the activity of the liver enzyme that breaks down caffeine, causing it to remain in the body longer.

The NHS advises pregnant women to keep caffeine intake below 200 milligrams per day, roughly two mugs of filter coffee. A 250-milliliter energy drink, for comparison, typically contains around 80 milligrams, although levels vary by brand.

What the Zebrafish Study Adds

A fish study can’t tell you what your next cup will do, but its underlying finding is worth understanding.

The University of Nebraska at Omaha research examined zebrafish with different behavioral patterns, including stress-sensitive and more exploratory groups. The researchers found differences in adenosine-related gene activity between those groups.

A small dose of caffeine did not increase anxiety-like behaviors across all fish. In stress-sensitive males especially, it appeared to reduce those behaviors.

The researchers emphasize that this is animal research, not evidence about caffeine’s effects on every person. However, the biological system involved is the same one that helps explain how caffeine interacts with the brain. The study does not suggest caffeine can treat anxiety. Instead, it adds another clue about why caffeine responses vary so dramatically between individuals.

What to Actually Do With This

For most people, caffeine is safe, though a few practical considerations can make a real difference.

Sleep researcher Dr. Lindsay Browning recommends paying attention to timing and avoiding caffeine roughly six hours before bed. However, she also notes that individual differences matter: “I’ll suggest to people that they might want to avoid caffeine after about 2pm if they are going to bed around 10pm. But a blanket hard-and-fast rule isn’t helpful, because it doesn’t really allow for the intricacies and the individualisation of what’s really going on.”

For many people, the body eventually provides a warning sign. Anyone who notices heart palpitations or a significant blood pressure increase after caffeine should discuss it with a doctor. But if coffee seems not to affect you, the answer may not be that caffeine failed. It may simply be that your biology handles it differently.

Does caffeine hit you hard, barely register, or does your sensitivity seem to shift depending on how much sleep you’ve had or what else you’ve consumed that day?